SIMPLE DISTILLATION TECHNIQUE

PURIFICATION METHODS

Typ e Cond i t i on s E xamp l e s

(A) Simple distillation (i) When liquid sample has (i) Mixture of chloroform

non volatile impurities (BP = 334K) and Aniline

(BP = 457K)

(ii) When boiling point (ii) Mixture of Ether (BP = 308K) &

difference is 80° K or more. Toluene (BP = 384K)

(iii) Hexane (342K) and Toulene(384K)

(B) Fractional distillation When BP difference is 10° (i) Crude oil in petroleum industry

(ii) Acetone (329) and Methyl alcohol(338K)

(C) Distallation under When liquid boils at higher (i) Concentration of sugar juice

reduced pressure temperature and it may (ii) Recovery of glycerol from spent lye.

(Vacuum distillation) decompose before BP is attained. (iii) Glycerol

(D) Steam distillation When the substance is immiscible (i) Aniline is separated from water

with water and steam volatile. (ii) Turpentine oil

(iii) Nitro Benzene

(iv) Bromo Benzene

(v) Naphthalene

(vi) o-Nitrophenol

Dist i l lat ion Techniques :

FR ACTIONAL DISTILLATION

STEAM DISTILLATION

DISTILLATION UNDER REDUCED PRESSURE

LASSAIGNE METHOD :1 . Nitrogen (N) : Na + C + N NaCN

(Sodium extract)Test : NaCN + FeSO4 + NaOH

boil & cool

FeCl3 + conc. HCl

Blue or green colour [Fe4[Fe(CN)6]3

Prussian blue

2 . Sulphur (S) : 2Na + S Na2S

(Sodium extract)Test (1) Na2S

Sodium nitro prusside

A deep voilet colourTest (2) Na2S + CH3COOH + (CH3COO)2Pb

Black colour (PbS)

3 . Halogen (X) : Na + Cl NaCl

(Sodium extract)

Test : NaCl + HNO3 + AgNO3

(a) White ppt soluble in aq. NH3 confirms Cl

(b) Yellow ppt partially soluble in aq. NH3 confirms Br

(c) yellow ppt insoluble in aq. NH3 confirms I

4 . Nitrogen and Sulphur together :

Na + C + N + S NaCNS

(Sodium thiocynate)

As in test of nitrogen, instead of green or blue colour,

blood red coloration confirms presence of N and S both.

5 . Phosphorous (P) :

P 2 2Na O

boils, Na3PO4

Solution in boiled with nitric acid and then treatedwith ammonium molybdate (NH4)2 MoO4

Formation of yellow ppt indicate presence of phosphatehence phosphorous present in organic compounds.

THE QUANTITATIVE ANALYSIS :

Quantitative analysis involves the estimation of percentage composition of various element by suitable method.

The molecular mass of the compound is also determined by suitable methods. The knowledge of molecular

mass and percentage composition help us to determine the molecular formula of the compound.

ESTIMATION OF CARBON AND HYDROGEN OR LIEBIG COMBUSTION METHOD :

Organic compound + dry, black CuO

(w g) Heat strongly in combustion tube

mixture of oxide

Pass through

(i) anhydrous CuSO4

(ii) anhydrous CaCl2

say increase in weight = a g

Pass through

(i) KOH (aq)

(ii) Saphnolite resin

say increase in weight = b g

For % of H - The increase in weight is 'a' g due to the formation of H2O

18 g H2O has H = 2 g

a g H2O has 2a

H g18

Since w g organic compound has = 2a

18 g hydrogen.

100 g organic compound has= 2a 100

18 w

g hydrogen

2 of H O formed2 weight

% H 10018 weight of subs tan ce

For % of C -

The increase in wt is b g due to formation of CO2

44 g CO2 has C = 12 g

b g CO2 has C = 12

b44

g

Since w g organic compound has 12

b44

g carbon

100 g organic comound has 12 100

b44 w

g carbon

2weight of CO formed12

% C 10044 weight of subs tan ce

Note : This method is suitable for estimation if organic compound contains C and H only. In case if other

elements e.g., N,S, halogens are also present the organic compound will also give their oxides which on

being absorbed in KOH will increase the percentage of carbon and therefore following modification

should be made.

(a) If N is also present : The gaseous oxide mixture is first passed over heated Cu gauze which reduces all

oxides of N to N2 which are neither absorbed by H2O nor by KOH

2NO + 2Cu N2 + 2CuO

(b) If halogens are present : A roll of silver gauze is placed at the exit end of combustion tube which retains

all the halogens on itself.

2Ag + X2 2AgX

2Ag + CuX2 2AgX + Cu

(c) If S is present : A layer of fused lead chromate is placed near the exit end, all the oxides of sulphur are

oxidised to PbSO4 & left in tube it self.

Cr+6 + 3e– Cr+3

S+4 S+6 + 2e–

(in SO2) (in SO4–2)

GENER AL CONCEPT FOR DRYING AND DEHYDR ATING AGENTS

(1) Acidic oxides ( non metals oxides such as CO2, NO2, SO2 P2O5 etc.) are absorbed by alkali (KOH,

NaOH, Ca(OH)2 etc.)

(ii) Basic oxides (Na2O, K2O etc.) are dissolved in acids (H2SO4, HCl etc.)

(iii) Acidic oxides can not be dried by basic dehydrating agent (e.g CaO) but they are dried by acid dehydrat-

ing agent (e.g H2SO4, P2O5 etc). However H2S can be dried by H2SO4 because it reacts with H2SO4.

H2S + H2SO4 2H2O + S + SO2

(iv) Basic substances can not be dried by acidic dehydrating agent but are dried by basic dehydrating agent

(e.g CaO). It is because NH3 is dried by CaO but not by H2SO4.

ESTIMATION OF NITROGEN

(a) Duma's method :

Organic compound CuO

heated strongly Mixture of oxides Pass over Cu gauze N2+ Other oxides

(w g)

Dry N2 drying agent N2 + Moisture

Pass throughKOH (aq.)

(Let Vml at N.T.P.)

22400 ml N2 weigh at N.T.P. = 28 g

V ml N2 weigh at NTP = 28

22400V g

since w g organic compound gives 28

22400100

V

w g

2Volume of N at NTP28

% N 10022400 weight of the compound

Note : This method can be used to estimate nitrogen in all types of organic compounds.

KJELDAHL'S METHOD :

This method is based on the principle that a nitrogenous organic compound on heating with conc. H2SO4

converts all its nitrogen quantitatively to ammonium sulphate ;

organic compound + conc. H2SO4 + K2SO4 + CuSO4

(w g) (raises the B.P. of H2SO4) (catalyst)

Heating in Kjeldahl flask

4 2 4 3add KOH

drop by drop(NH ) SO NH is formed

Pass this gas through V1 ml of N1 H2SO4

Now titrate the H2SO4 by another alkali say of N1 H2SO4 (NH3 is absorbed here)

V2 ml of N2 ,KOH are used

Meq of H2SO4 taken = N1V1

Meq. of KOH used for H2SO4 = N2V2

Meq. of H2SO4 for NH3 = N1V1 – N2V2

Meq. of NH3 formed = (N1V1 – N2V2) or say NV)

(the difference obtained or V ml of N H2SO4 are used to absorb NH3)

3NHW

1000 NV17

3NH

17W NV

1000

Now 17 g NH3 has 14 g Nitrogen

17

NV1000

g NH3 has 14 17 NV

17 1000

g Nitrogen

= 14NV

1000 g Nitrogen

w g organic compound has 14NV

1000 g nitrogen

.100 g organic compound has 14NV 100

w 1000

g nitrogen.

=1.4NV

w g nitrogen

Normality of the acid volume of the acid

% Nitrogen 1.4weight of the compound

Note : This method is simpler and more convenient and is mainly used for finding out the percentage of nitrogen in

food stuffs, soil, fertilizers and various agricultural products. This method cannot be used for compound having

nitro groups, azo gp. (–N = N–) and nitrogen in the ring (pyridine, quinole etc.). Since nitrogen in these

compounds is not quantitatively converted in to ammonium sulphate.

ESTIMATION OF SULPHUR (CARIUS METHOD) :

Organic compound + con. HNO3 + BaCl2 ppt (BaSO4) is dried and weighed say "a" g

(w g)

S 3Furning.HNO H2SO42BaCl BaSO4 + HCl

233 g BaSO4 has 32 g sulphur (a g)

a g BaSO4 has 32 a

233

g sulphur

% of Sulphur = 32 a 100

233 w

4weight of BaSO formed32

% S 100233 weight of compound

ESTIMATION OF HALOGENS ( CARIUS METHOD) :

Organic compound + con HNO3 + AgNO3 ppt (AgX) is dried and weighed say a g

% of Cl = 35.5 weight of AgCl

100143.3 weight of compound

% of Br = 80 weight of AgBr

100188 weight of compound

% of I = 127 weight of AgI

100235 weight of compound

ESTIMATION OF PHOSPHORUS - (CARIUS METHOD) :

Organic compound + con. HNO3 + Magnesia mixture (MgSO4 + NH4OH + NH4Cl)

(w g)

2 MgNH4PO4 Mg2P2O7 + 2NH3 + H2O

Precipitate is dried and ignited to give magnesium pyrophosphate (Mg2P2O7) weighed say a g

2 2 7weight of Mg P O formed

weight of compound

62% of P 100

222

Atomic weight of P = 31 × 2 = 62 g

molecular weight of Mg2P2O7 = 222 g

ESTIMATION OF OXYGEN :

There is no direct formula and method for estimation of oxygen however, percentage of oxygen is calculated by

% of oxgyen = 100 – [Sum of the % of all other elements]

Note - A direct method for estimation of oxygen has been given by Alusive in 1947 known as Alusive

method.